Parallel LED Array For Elegoo Mars: Higher Precision, 3-4x Faster Printing

As you may have noted from my blog, I tune Elegoo Mars for better performance as my hobby. After the complete rebuild of the Z-axis, making it quieter, more-precise in XY direction, and easy print removal, I focused on printing speed.

I have already experimented with making the screen monochrome, however, due to lack of resources (dead screens) I abandoned the idea. The second way is to tune the light source. You can already buy LED lamp replacement, but what hobby would it be if just bought everything? Also, their parameters are not as good as my aims.

One disclaimer – the whole build was performed during the lockdown period during COIVD-19 outbreak, so many manufacturing and supply options were limited. Therefore, some of the approaches presented in this post are rather hacky and I would make them differently during normal times.

I decided to build a 7×4 LED array. The reason for going for such a high number of LEDs is to limit their radiation cone – the goal is to produce as parallel rays as possible. With fewer LEDs, I would need a wider cone. When you have rays under an angle, it leads to a side exposure and imprecisions in the print. I used LG6565 – 10W LEDs which emit peak wavelength 405nm. That’s 240 W. Original Mars is roughly 30 W.

I created a simple PCB. I used copper-clad with an aluminium substrate to provide better cooling. Normally, I would buy sodium persulfate and etch the board. However, during COVID outbreak all the drug stores were closed so I just cut the board with an X-acto knife and peeled the excessive copper. Soldering the LEDs on the aluminum substrate was not an option and reflow oven was out of the reach – so I just cut pieces of solder wire, put them on the PCB, put some flux on it, and put it on a gas stove on a thick piece of steel. After 2 minutes, the solder melted, the LEDs nicely arranged. After attaching some wires, the PCB was ready. To improve the cooling I also mounted a beefy heatsink to the backside of the PCB and I added two 60 mm quiet fans.

To drive the LEDs, I decided to go with a separate power supply. I used a switching boost constant-current DC/DC converter bought on Aliexpress. I decided to power it from 24 V to make it as efficient as possible. I added a MOSFET for switching the LEDs. To make things simple, I attached wire on the gate of the original on-board MOSFET for LED switching. One note – the LED array is really bright! There was also a lot of heat coming out of it – but not the excessive heat in the heatsink – the blue light itself carries a lot of energy and you could feel warm even when you put your hand nearly meter from it.

This was the basic prototype. Next, I focused on making the light rays parallel. First, I started with designing a mounting bracket, which features ribs shuttering the light from neighboring LEDs. It limits the angle to roughly 45 °. I printed in 2 pieces on my Mars. I painted the insides of the ribs with Black 3.0 to limit the reflection.

Then I decided to create custom lenses to make the beams parallel. I wrote a simple Python program to calculate the shape of an aspheric lens, which in theory should make the beams parallel.

I decided to cast the lens from clear epoxy. First, I measured that it has roughly the same transparency for 405nm light as glass by casting a test piece and using a photoresistor and 405nm laser pointer. Then I measured the refractive index of the epoxy by shining the laser pointer in it and drawing the light beam path on a paper underneath the test piece. Then used the program above to create a spline sketch of the lens, created a 3D model in Fusion 360. Then I offset the lens surface by 0.05mm and printed it on my Mars using Siraya Tech Fast resin. After printing, I mounted the lens in a hand drill and used sandpaper (grid 400, 600, 800, 1000, 2000, and 4000) to polish it. To finish it, I used a fine abrasive paste to create a glass finish.

Having the shape of the lens, I created a silicone mold and cast the lens out of the epoxy. To my surprise, it worked! The lens was nicely transparent and few experiments with my headlamp showed it does what it is supposed to do – when you put LED in the focal point, it produces a nice circle of light. To verify the parallelity of the beams, I pointed it at a wall roughly 2 meters away. On 2 meters, the casted circle had a diameter of 100 mm. That means that from the initial 45 ° cone of radiation it went to 4 ° – not bad for a homemade lens from scratch!

The next challenge was to create 28 of them. Since the epoxy cures 48 hours I decided to create multiple silicone molds, put them in a frame, and cast a single piece lens. I struggled a little with capturing large bubbles inside the mold, but after two fails I cast a usable piece. Well, sort of. The individual lenses work, unfortunately, the boundaries between them have a lot of artifacts. Therefore, the result was not good.

I run few tests prints – with and without the lens. My observation were the following:

  • the 0.05mm layer cure time for Siraya Fast and Elegoo Gray was 1 second
  • the lens caused some blind spots – which was expected,
  • surprisingly, I observed much less exposure bleeding without the lens compared to the original Mars light source,
  • there was no significant difference in exposure bleeding with and without the lens,
  • the resin was easily able to reach 50 °C – the amount of light energy carried in the light is really high. In such conditions, even the thickest resins are nicely thin and practically water-like 😛
  • I overestimated the LED cooling and the heating never got hotter than 5 °C over ambient temperature,
  • with such short exposure times, build platform peeling and retraction takes most of the print time – you expose in a single second and then it takes 5 seconds to peel the layer…

My first impressions were good. The mod was working and I was getting nice prints. I didn’t need a heater, actually, I was considering building a cooler for the build area. However, after 10 printing hours, I started to observe weird print failures. The models did not stick, were soft, and broke often. Increasing the exposure time did not help. I noticed, that the LCD is not as bright as it was. It was actually pretty opaque, but there were no dark spots on it like in the case of “common LCD failure”. Probably the heat killed it.

Therefore, I decided to cut the LED power in half (120 W) and to add an extra fan, which blows the air from the side of the array and therefore, creates a constant airflow below the LCD which cools it. After this mod, my exposure times are around 2-3 seconds per layer and the resin stays cools. Even it is double the original value, the printing times are reasonably low as most of the time is consumed by layer peeling. With this mod, I printed over 150 hours with a single LCD and it serves me well. I do not observe any damage except the damage I caused by accidentally scratching it. Note that it roughly equals to printing 600 hours on unmodified Mars.

To sum it up – you don’t need monochrome when you have enough power (and enough cooling)! Well, to be more serious – this was an interesting experiment and I am personally happy about the results. The mod itself is rather hacky and therefore, I do not publish any materials to reproduce it. It was a nice fun project during COVID lockdown, but it is not suitable for people to reproduce it. Currently, I am out of time and energy to invest into finishing this mod into a reproducible form. This is also why this report is quite brief and not-so-detailed as I was originally aiming for.

PS: It makes no sense to make commonly available lenses! Buy them! I made the lenses only as challenge and a hobby. It (kinda) works, but probably is not worth your time.

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